Monitoring a large-scale wireless sensor networks (WSN) is very difficult, not only because it is large and complex, much of difficulty comes from the lack of visual analysis tools. This paper describes SNDS (Sensor Network Distributed Sniffer),a distributed monitoring and protocol analysis system for large and complex sensor networks. SNDS is based on the use of sniffers co-deployed with the target sensor network. We mainly study and solve the following difficulties: monitoring sensor networks with large traffic, synchronizing the time among the distributed sniffers accurately, analyzing the protocols efficiently and flexibly. We test the stability and time synchronization accuracy of the whole system. The results show that the SNDS has the capability to work stably, synchronize the time accurately and analyze the wireless protocols efficiently.